There is a continual need for low cost, small sized, fast responding sensors for environmental toxins, industrial chemical toxicants and chemical warfare agents. In addition, there is a growing need for sensors that can monitor the residual adsorption capacity of activated carbon filtration cartridges in gas masks and personal protective equipment. In the United States, draft government health and safety regulations require the detection of VOCs prior to depletion of the carbon bed's adsorption capacity. However these regulations have not yet been enacted due to a lack of suitable sensing devices. (NIOSH, Certification Criteria 2005, Procedure No. RCT-APR-STP-0066; OSHA, Vol. Standard 1910.134(d)(3)(iii)(B)(2), U.S. Department of Labor, 2006) These sensors would operate by detecting organic vapors breaking through a filter bed of activated carbon.
A number of colorimetric, thermal, and electrical sensors for VOCs have been deployed for carbon filtration bed end-of-service-life applications, each with limited success. Dye-based, passive colorimetric indicators are functionally limited by chemical group selectivity, restricting their utility as broad-class sensors for hazardous compounds like VOCs. Many calorimetric sensors are additionally irreversible, adding to implementation cost.
Thermal sensors measure the local temperature of activated carbon which can rise by several degrees from heat released during adsorption of VOCs. However, thermal sensors for adsorption events are difficult to implement outside of a controlled laboratory setting and are ineffective at detecting the adsorption of small quantities of analyte over longer time periods.
Transconduction sensors measure changes in electrical conduction that result from adsorption of analyte into a conductor or semiconductor. Metal oxide transconduction sensors require large currents to operate at elevated temperatures, and recent chemiresistor and semiconductor transconduction sensors contain supporting electronics that are susceptible to environmental stress. Typical transconduction designs retain a large form factor that is difficult to use inside a respiratory mask carbon bed.
Fiber-optic-based sensors have been applied to a variety of remote sensing problems: measurement of pressure, humidity, vapor-phase chemicals, and aqueous biomolecules are leading examples. With a width of only a few tens to hundreds of microns, these sensors are impervious to electrical interference, require little power to operate, and can be multiplexed together into distributed sensor configurations.
Many fiber optic sensors for organic and water vapors operate by detecting changes in transmitted light intensity. An example of this intensity measurement technique is use of the fluorescence enhancement of an immobilized polymer to detect for organic vapors. See, S. M. Barnard & D. R. Walt, “Fiber-optic Organic Vapor Sensor,” Environ. Sci. Technol. 1991, 25, 1301. Another example is the immobilizing of cobalt chloride on a plastic fiber to sense for water vapor in a calorimetric reaction (See, C. M. Tay, et al., “Humidity Sensing Using Plastic Optical Fibers,” Proceedings of the SPIE 2004, 5590, 77.) An additional example is the sensing changes in organic solvent refractive indices with surface plasmon resonance at a fiber terminus. (See, H. Suzuki, et al, “Development of a dual-color optical fiber SPR sensor”, Sensors, 2005 IEEE, 2005, 865. Fabry-Perot and interferometric sensors have been developed that utilize optical interference effects for optical transduction. These sensors monitor transmitted light intensity induced, for instance, by mechanical changes in a fiber by temperature or pressure. See, e.g., B. Lee, “Review of the Present Status of Optical Fiber Sensors,” Optical Fiber Technology 2003, 9, 57; A. Wang, et al, “Fiber-Optic Temperature Sensors based on Differential Spectral Transmittance/Reflectivity and Multiplexed Sensing Systems,” Applied Optics 1995, 34, 2295; F. Mitschke, “Fiber-Optic Sensor for Humidity,” Optics Letters 1989, 14, 967.
Fibers that were made porous have also been published in the literature. A sensor that coupled a porous fiber with bromothymol blue to monitor transmitted light for ammonia detection has been reported. Q. Zhou, et al,. “Porous Plastic Optical Fiber Sensor for Ammonia Measurement,” Applied Optics 1989, 28, 2022. Another study coupled a ruthenium complex to a porous fiber to sense for oxygen by fluorescence quenching. B. D. MacCraith, et al, “Fibre Optic Oxygen Sensor based on Fluorescence Quenching of Evanescent-Wave Excited Ruthenium Complexes in Sol-Gel Derived Porous Coatings,” Analyst 1993, 118, 385.
Previous inventions from the Sailor research group at the University of California at San Diego concern the construction of millimeter to micron-sized, nanostructure photonic crystal particles that respond to various organic and inorganic molecules in the vapor phase, including volatile organic compounds and chemical warfare agents. The response of these materials is observed in the form of a characteristic change in the reflection spectrum from the photonic crystals.